Subterranean deposits of coal, whether of “hard” coal such as anthracite or “soft” coal such as lignite or bituminous coal, contain substantial quantities of entrained methane gas. Limited production and use of methane gas from coal deposits has occurred for many years. Substantial obstacles have frustrated more extensive development and use of methane gas deposits in coal seams.
One problem in producing methane gas from coal seams is that while coal seams may extend over large areas, up to several thousand acres, the coal seams are typically fairly shallow in depth, varying from a few inches to several meters and have a low permeability. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits are not amenable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced, further production is limited.
Another problem in producing methane gas from coal seams is subterranean water which must be drained from the coal seam in order to produce the methane. As water is removed from the coal seam, much of it is replaced with recharge water flowing from other virgin areas of the coal seam and/or adjacent formations. This recharge of the coal seam extends the time required to drain the coal seam and thus prolongs the production time for entrained methane gas. For example, in Appalachia, it may take four or five months of pumping water from a coal seam before the recharge water head pressure has dropped to a point where gas can be produced. When the area of the coal seam being drained is near a mine or other subterranean structure that reduces recharge water by itself draining water from the coal seam, methane gas may be produced from the coal seam after a shorter period of water removal.